Swapping out larger fish for smaller usually isn't a problem. And your 125 is large enough that unless you go completely overboard you should be able to keep a good number of fish.
Live rock and base/live rock are pretty much the same. Once any "dead" rock is seeded it will eventually become live rock. Marco rock is a perfect example.
Just remember, when you add new rock to your system it will cycle and that's very hard on your livestock.
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250 gal display tank plumbed directly to my wallet
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