awetmore, you shouldn't have to adjust SFM much for machine size, but you can get a bit more tool life if you set the G-Wizard machine profile to use 80% of recommended SFM.
Chipload will be the one that's messing with you, in all likelihood. Even there, it may be more a matter of knowing what the max depth of cut you can do on your machine is for full slots than trying to dial back the chipload.
The problem with dialing back the feedrate and chipload is eventually you will have so little chipload it causes rubbing and tool life is a lot shorter. Beware chiploads that start to be measured in tenths unless they're pretty darned small cutters and very sharp.
For a full slot, what symptom are you getting? What sized cutter and parameters are you using?
If it is just poor surface finish, depending on that finish, you may want to consider doing multiple passes. This is not uncommon even for larger mills. There's quite a bit of cutter deflection on a full slot. So where possible, and where surface finish matters, take it down the middle, and then climb mill the two sides.
Lastly, the biggest issue a lot of folks have is not keeping the chips cleared. Even if you don't have flood coolant, but an air blast on it and make sure it is blowing the chips up out of the slot. Chip recutting is really hard on tool life as well as on surface finish.