Happy Birthday!
As for the SQ fluids.... usually it's a matter of how much gets in how fast. Sometimes their little hearts can't deal with the excess all at once. Also, the skin itself taking on huge amounts underneath can be a problem, but rarely.

I don't remember how much Maggie (aka "tough as nails" ) weighs, but if she's 15-20 lb, she can have 200 at once without too much trouble. I might spread it between two different sites to lessen the stress under the skin. Lisa's idea of trying 150 first is good.

I'm so glad she's doing this well.
Enjoy!
Jes