Okay well if you are after the current 'in-things' the Rogue or Goon are both popular and they're both 24mm, I am thinking about these but they're bigger. I use the SMPL mod which is 22mm because it's small for out and abound has a good button which is recessed and won't auto fire (the problem with mechs from the past which had to be locked).
Normally people use drippers but if you want a tank it really should be small (serpent mini size) or it will look ungainly and easily be knocked over. The tanks I use with a mech is the Nextasis/Netiny with are mesh genesis or an RDA. The main thing is to do a bit of research and find a tank/dripper that will suit the mod in every respect looks are important with tube mods.