My circuits are supplied by a (actually 2) 50A supply and work beautifully. However, when something is wrong (e.g. a short) the smoke get really thick.

So, if your circuit is without shorts or other forms of mistakes, you can, should you so desire, use a 1000A supply with no risk. I guess your findings with the 2A supply show that there is no risk ("it kind of works, just not 'firmly' when too many relays are on").

I just got curious: what are you switching with the relays, 6W relays seem awfully large for most things.

Erik