Hi Barry.
I will try to respond even though I'm not that much of an expert in IE3000 series.
As far as I can determine you are right about the possibilities for IE3000, i.e. you only have two 10/100/1000 port per switch. This leaves you to buy two switches if you need to have more uplinks.
To the best of my knowledge the CAB-SFP-50CM= only works with the Cat3560 series.
Also, interconnecting two switches via RJ45 will consume one of the two 10/100/1000 ports, as they are dual-purpose ports, i.e. you can either use the RJ45 or the SFP.
So it seems that you will either have to live with only 100 Mbps fibre for one location - or perhaps if you have enough fibre you could create an etherchannel.
I have no idea about future plans for IE3000, but it would seem that a 4 port 10/100/1000 could be handy.
HTH.