I think you can add them no problems cories and rams usually work really well together and you will be able to add in a shoal of maybe 6 small tetras or similar as well. Rams are probably one of the top 10 feature fish for a 15g tank IMO. I can accept what the Fubar is saying about a 2 foot territory and I wont argue how ever its usually documented that rams (among other dwarfs) take up around a 12inch square of territory in a tank. However as they are the only cichlids this will only be a problem if they breed really, unless they do stake out a "home" but all that will happen is the corys get chased out. Bolivian rams are usually a bit more forgiving than the blue rams as well so I wouldnt worry at all.
So far as the inch per gallon rule goes its a good guide but by no means is it a definitive rule. Again no disrespect to Fubar but I would never take into account decor substrate filters etc to take off liters. You have a 63 liter tank so you should always fill it to the fill line as well.
I dont think you will have a mini cycle from adding 2 rams either. But still keep an eye on it but I would be quite surprised if it happens.
Wills