My current 250gallon L shaped tank was made using an old 70 gl. fused with custom 180 gl. sized pieces of glass.
I had to join 2 bottom pieces, so I used the spare glass sheet of the 70. gl tank to reinforce the bottom seal. I used almost 1 whole tube tube of silicone in that joint.
Also, the back panel has 2 pieces of glass glued together in a straight line. The original plan was to reinforce them with a piece of glass in the back, but for some reason I forgot to do that, and the silicone bead has been holding like a champion. However, this tanks has a lot of reinforment crossbars covering the entire perimeter and across. Also, it looks ugly and I have a plastic plant covering that zone.
Separating the glass was also difficult. I used 3 different knives to help me, and also the glass got a little cracked in the edges.
Perhaps the smaller 10g. tanks would be easier to work with, but, honestly, it would be easier and cheaper if you go to the glass shop and buy some brand new glass sheets and biuld a new tank.