I think the bottom of your tank is stocked fully already. If you didnt have the BN plec, then maybe you could. Just keep a close eye on him with the cories. Wait, do you want a pair or just one?
I tinhk you'd be fine - my only fear is that weather loaches get quite big and rough, and might 'steamroller' the blue rams.
Blue rams are pretty innocuous - a single is simply not going to threaten anything else in the tank. It will setup a very small territory if it's a single
I too don't think that one is going to pose much of a threat to other bottom-dwellers. On there own they are pretty laid back and quite more tolerable of other fish around it's teritory.
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