New to Kitbashing

Before answering your questions I'd just like to say that you are off to a great start. The figure in the main picture has a good movement to it and feels like a natural pose.

1. If you don't already keep a bits box. This should be where you keep spare parts from sprues or that you have cut off any models you have been kit bashing. I also chuck in things that might be useful that would otherwise end up in the bin, wire, mesh, tubes, scraps of plastic packaging that can be repurposed, just whatever you think might be useful.
2. I use unscented Dettol. Leave plastic or metal in it overnight and use some fresh Dettol and a toothbrush to clean them up. Just be sure to put plenty of paper down underneath as the paint gunk dries and can pretty much only be cleaned up by scrubbing it with more Dettol.
3. Looking at your other comments I would also recommend the Howling Griffons, but if you have a colour scheme that you think would look cool just use that and make yourself a nice successor chapter. Personally one of the things I have always enjoyed about 40k is being able to make up your own lore.

One last piece of advice if you plan to do more sculpting with putty. Try getting your hands on some greenstuff, it is my modelling putty of choice and gives the best finished product in my opinion.

Any general advice for kitbashing?

This one here looks pretty fully realized, but in general my advice is don't forget about scratch building components. Jewelry chain, zipties, plastic scraps, and all other sorts of bobbles can help finish out a kitbash.

How do you strip your old paint? It took multiple washes with simple green and scrubbing, which isn’t what I have heard is the general amount needed.

Personally I use Super Clean, but there are tons of viable methods. Here's Goobertown Hobbies trying a bunch.

What I've learned from many eBay rescues is that sometimes you can't or don't need to strip back to the bare plastic. As long as the mini's surface texture looks reasonable, spray paint covers many sins.

What chapter do you think would go well with this guy? I wanted to keep some of the yellow and maybe red from the original pieces.

I think the Firehawks look pretty cool in either version of their colors.

1. Guitar strings or rolling green stuff against a plastic comb are great means to make all those extraneous external wires typical to 40k.
2. Multple passes with letting it bathe simple green then washing afterward sounds a bit atypical but not unusual. Very often I don't strip at all and just paint over it.
3. Dark Angels.