Is using a single 25MHz oscillator feasible

The only potential issue I have had was deriving the right frequencies; that's why I usually have two slots, one for a 50M and one for a 48M... they can get you different time bases. If you can get all of your frqeuencies with the one, then you should be ok. The only case I could see forming a problem was if you needed multiple output clocks from the FPGA; you would ideally use the outputs of a PLL for these clocks, and theres only one external output per pll... so keep that in mind too