Coastal hay would be fine to feed her, just another grass. You may want to give her small amounts for the first few days, just to make sure there are no digestive issues with it. It doesn't hurt to give them a variety. I'm feeding my babies(I guess they're considered juniors now) a mix of alfalfa, timothy, and another grass hay that I think is coastal. Variety for rabbits is actually good too, as it helps with the wearing down of their teeth.
Sherwood recommends switching babies to the adult food at 12 weeks. If Ellie really is 8 weeks and just small for her age, if you haven't already ordered the food yet, you could probably get away with just starting her out on the maintenance/show pellets, then you wouldln't have to do a transition in a few weeks, from the baby pellets to the adult. The adult pellets are a mix of timothy and alfalfa and have 14% protein which is only 1% less than the baby pellets, so she would still be getting plenty of protein, plus you are feeding her a little alfalfa hay which will give her some added protein. As long as she seems healthy and is gaining a bunch of weight each week, then you are doing just fine. The recommendations on what to feed aren't an exact science and sometimes require a little tweeking along the way. Like, it's recommended to feed unlimited alfalfa pellets for rabbits under 6 mo., but I've found that some of my babies had poopy bum problems with unlimited pellets, so now they get limited pellets and have done much better with that. Unfortunately it can come down to a little trial and error with rabbits, and just trying your best.