Question: Is learning (and playing) the smallpipes the same as learning the standard bagpipe? That is if you can (learn)/play one, can you then (learn)/play the other?
I would say yes and no. Basically the fingering of GHB and small pipes are the same. However, usually SP chanters in D have smaller hole spacings than the average A chanter in either GHB or SP. It is very possible to learn a SP to GHB and vice-versa. In my experience and with others...the transition from GHB to SP is a lot more "forgiving" and the learning curve not so bad; whereas SP to GHB can be problematic at best. Now...this is assuming that the SP is mouth-blown, going over to GHB will not be too much of a "shock", but like anything else having the proper instruction is prudent.

Either transition from GHB to SP or vice-versa that involves a bellows-blown SP will require learning a whole new set of rules. They are entirely different disciplines that must be learned separately depending on which direction you're coming from. When I went from mouth-blown SP to bellows, it took me a month or so (1 hour a day practice) to get into the groove sort of speak. The timing with "both" elbows, bellows and bag pressure took some doing, but I achieved a decent technique to where I could play with ease and comfort.

Also from my experience, getting into "frame of mind" on which instrument I will be playing also can play a factor. I usually never play GHB and SP right after the other. Really screws up my timing. I usually wait an hour or so...then do it. For me....if I play the SP (bellows) for awhile, quit and then pick up the GHB and try to play one tune...."pandemonium" sets in. My brain is still in SP mode. Wait awhile and everything is fine. Weird? Yes. But I guess that is the way I am "wired". Many other players I have discussed this with say the same in some way. Not everyone has that problem, but for me, a little break in between does help.

I usually pick a "day" throughout the week and just play that instrument and not the other. Just remember, GHB basically has a "higher set of standards" when learning whereas the SP can be played a whole lot differently. They are two different disciplines in learning and playing. Just remember that.