I assume there is a distance issue that requires fibre, otherwise you could use cheaper 1000baseT switches (e,g. 3750).
You have 3 GBICs in the 3550-12G. I would try and connect all 4 downstream 3550's directly to the 3550-12G core switch so that you maintain a constant switch diameter.
Note that you can also run dual links from the 4 access switches back to the core for the cost of some additional GBICS and fibre patch leads.
How come the two 3550-24-SMI switches in the middle of the access layer have three GBICs? Are you planning on linking those two directly together (if so, don't).