Using two different types of salt mix is fine. However, I would not use anything besides ocean pure pro, reef crystal, or instant ocean. These salt are pretty consistant and produce better water parameters.
Mangroves are not great for exporting nitrate; chaetamorpha works much better. Try searching -- because there are many threads and experiments between these two macros. Also, mangroves are higher maintenance. The leaves of a mangrove will leave behind salt build up which contains all the...
IMO, it is not a good deal at all. I would offer $300 for the tank and sump only. However, if you can't find a new or used stand for this tank then I wouldn't touch it at all. Good luck!
The frogfish will eat anything its size incuding your shimp. Frogfish will leave your corals alone, but they are known for perching on corals which may stress out the corals.