Sierra - i agree, get the biggest you can afford. there are tons of tricks to aquascaping. you can make 60 pounds fill a 100 gallon tank (depending the demensions) or you can make a 100 gallon tank look empty with 150 pounds. some of the rocks are pretty dense and some are very porous. i had a 29 gallon tank and upgraded to a 75. i added a 25 gallon package and my tank looks full now.
i still would have ordered a bigger package, had i been able to afford it. i can see all the guys/gals that got bigger packages than me have tons more life/diversity. if you are interested i could show pics of all the stages. i have pics of the 29, the 75 with the rock from the 29, then with the TBS added later.
as far as adding TBS later, its a fine idea. if you get a decent size package, it should handle the small cycles that come with adding more rock later. also, you wont have to order a package later and deal with sand and critters, you could just buy their rock. or if you do buy a small package you can add the sand carefully, but the base rock will end up on top of your previos order. i always enjoy a full on re-aquascape though
the options are endless. just be prepared with every order to do lots of water changes, and keep salt and water in mind when factoring the costs. you would hate to spend all your money and watch the life die because you cant get salt

i wouldnt think that would happen, but you never know
good luck and keep us posted!!!