To answer your first question the answer is NO! Do not mix Tamiya, which is an alcohol-based Acrylic; with Humbrol, Airfix or Vallejo paints as these are water based, If you do mix them you'll end up with a thick gooey mess that’ll be useless for anything.
Acrylics come in two variations, like Airfix, Humbrol and Vallejo are aqueous (Water) based and can be thinned and cleaned with plain tap water, although it’s always best to use the specific manufactures thinners for thinning the paint. I use Vallejo paints and have no problem in using them with a brush or airbrush if thinned. Same with Humbrol and Airfix paints, although the paints supplied with the various Airfix kits is usually of dubious quality and very thick. Most modellers throw it away, and use a better-quality paint. You say you use a brush; I hope your brushes are of a good quality, you’ll need decent water colour brushes, and if doing large surfaces, you’ll get better results with a reasonable wide flat brush, 8 to 15 mm wide should cover most uses for 1/72nd to 1/48th scales, and bigger brushes for bigger scales.
Tamiya paints are alcohol based, and can be cleaned with water but you’ll need the Tamiya thinners to dilute them otherwise they’ll go thick and lumpy. Meths and or methanol or I.P. alcohol can be used if you know what you’re doing, but I’d still recommend using Tamiya’s thinners. Personally, I have problems brush painting with Tamiya and usually only use them for small items and airbrushing.
One thing you’ll find with Acrylics is that they dry very quickly, this is especially so where I live (Cape Town). I always put a drop of retarder in the mix when I use any acrylics, this helps with not only the rapid drying, but also improves the flow without altering the covering power, although it can leave the dried paint with a slight sheen..
I’ve never used Mr Colour or AK (Which I believe is make also Meng paints.) but I believe they are also alcohol or solvent based, and therefore they should only be thinned with their own thinners. (Try smelling the opened bottle, you’ll soon smell any alcohol solvent.) I believe AK and Meng are very thin pains and could give problems with brush painting being mainly used for airbrushing. You might want to check out Flory Models web site, as over the past week-or-so they’ve been discussing these paints and how to use them. Also, there are many painting tutorials on the various web sites.
Changing from enamels to acrylics can be a bit of a learning curve so some experimenting might be called for, try and use an old model to experiment on before you commit to your current or new built. Good luck and hope this help’s, and would love to see some of your results in the future
Remember we do this for fun. John the Pom