Hello,
for ~20 mm mechs the Reaper Black Bones IMF Bulldog is my first choice. Close hatch for 20 mm + 28 mm, 28 mm pilot head included, 20 mm PSC tank commanders will fit also. Reaper offers small robos and toolbots. All black bones stuff.
Big mechs, warhammer 40k , there are some mechs. A different paint job for the "wright" modell will do. Infinty, maybe Warmachine, Void had mechs, still aviable.
Reviresco Starguard have big Robos for ~20mm. They are nice.
The battletech mechs are nice, but mostly to small for ~20 mm pilots.
For my skirmish games, 3 mechs + 30 soldiers per side, small and medium mechs are big enough. Real heavys are out of that kind of game.
Yours, Hermann