Anything bad with mixing weapon types?

I wouldn't mix. Your half sheep, half wolf. You are only a half threat for any class with this build.

For Ancilelot: don't go alone and keep shooting it even with plasma. His shields will break.

I understand the thinking. In theory it would be good to be able to damage Ancile and physical shields with one setup, but unfortunately you actually just dilute your effectiveness against both. This is the stage of skill development where you need to learn to be selective about your targets. If you have rockets you should be looking at the britbots (except Ancilot) and Rhino as if they are a meal. If you are in a plasma setup, you should be prioritising Carnage, Fujin and Ancilot (preferably when he's distracted/turned away). If you are a plasma setup hunting Galahads, you are wasting resources.

The Thunder weapon can be used effectively with both rocket and plasma setups, and the Thunder Magnum Leo or Thunder Pinata Leo are good examples of this. They are excellent setups against Ancilot. Thunders strip down Anciles VERY quickly. If you have the Thunder Pinata setup, make sure you fire the Thunder first before using the Pinatas. Once it's down you can unleash your rockets if you have them, or allow your teammates to take him down. The Thunder Mag Leo is one of my favourite setups. You should stay between 300 - 350m of any target to avoid Orks and Pinatas. Also try to get the Leo to at least lvl 9 as soon as you can. Then you can focus on levelling the Magnums.

Don't forget to cornershoot with the Mag Leo, particularly if you have to engage a Plasmahad (always cornershoot WITHOUT using your Thunder). You can press the two Magnum buttons in quick succession, then hold, then it will only shoot these two weapons consistently rather than wasting your Thunder by using the red alpha strike button. If you start to wear down their shield while they approach you, you want to do it behind cover where you can chip away at the shield without receiving any damage yourself. Always take advantage of getting free damage to a shield. He will either run away or chase you, in which case you have a fresh Thunder waiting for him at point blank range once he comes round the corner. This is the best way to strip their shield as fast as possible. In general you should avoid Gareth and Galahad in a Thunder Mag Leo, but it's not always possible and if you have to engage them this is the way to do it. You should prioritise Ancilot though, preferably paired up with a teammate. His shields are MUCH weaker than the other Britbots, and between your Magnums and Thunder they should break without too much trouble as long as he isn't maxed out.

Hope this helps,

DV

I think we all go through what you're describing at some point. My best advice here is to make a plan of what you want long term, and build your bots towards that goal. Don't upgrade weapons you absolutely won't be using later, but don't be afraid to buy some extra thunders, pins, pinatas,or tulumbas to fill up weapon slots for the time being. I would leave your PDB Griffin as a full set. Leo can easily be used as a thunders/pin or thunder/pinata right out of the box with him at level 6 and the weapons at 5. It won't be top damage, but you don't waste valuable time upgrading weapons or bots you won't use later. RDB Griff out of the box is particularly effective and reasonably cheap. 

Without knowing your hangar specifics/tier, it is hard to give sound advice as to what order to do things in, or what purchases might be worth making. I attacking an ancilot with a PDB griff, flanking an already engaged one is preferable.

Mixing ranges is OK as long as the range disparities isn't more than 50m.

Some folks think using a mixed setup is a "counter" to everything when in reality it halves weapon effectiveness. Direct targeting with plasma is not possible while leading rocket fire. Inevitably you are either firing only one weapon at a time or worse, you fire both and miss with one set. The same principle is in effect with large range disparity. Half your weapons are out of range or you are closer than you want to be with your weapon type (example Taran hydra Griffin from 350m).  The only exceptions here for mixed weapon types are ones with compatible lock-on features and similar range (aphid pairs well with both rockets and plasma).

It is for these reasons that I would steer clear of mixed weapon types and follow DarkVagabond recommendations. Use weapons with compatible range and targeting requirements and compensate for enemy counters with better target selection. 

The only real successful build I can think of off the top of my head is the stuka, aka aphids and orkans on a Griffin. The rest, people defend but don't hold much merit.

As far as mixing plasma with splash, it's a catch-22. It also depends on your league. Once you get to Diamond (iOS) or upper Silver (Android), most reds you see will either have Ancile or physical shields. This means that against most of the bots you're going to face, you're only half as effective as you could be.

Some will say, "it's better to be 50% against everyone rather than all or nothing". That's not an accurate statement though. When you're 100% effective against some, and 0% effective against others, you can make your gameplan to attack those 100% targets and avoid the 0% targets. When you're 50% against everyone, you can't gameplan for that. You're simply 50% as effective as you could be, no matter what you do - only luck can help you, as in getting lucky enough to run into a lot of bots with no shield at all.

I believe the above addresses your question, but I'll touch on ranges a little as well. Mixing ranges generally isn't a good idea, sometimes even 50m differences. The Taran/Piñata setup is a good example. There's no reason to not start firing Tarans at 350m, but to do so and not waste your Pinatas, you have to click both side buttons quickly without accidentally hitting the red button or either of the pinatas. It's possible, but adds a level of difficulty you shouldn't have to deal with, and doesn't have any tangible benefit to compensate for that. Thunder is a benchmark weapon that you can mix with any short ranged weapon, because it really serves its own purpose, and becomes more effective as you get closer.

Basically, any time you're mixing ranges, you're going to have to be good at firing individual sets of weapons and then hitting the red button at the right time. So ask yourself - does this setup provide an advantage that makes it worth the difficulty using it?