Well, the reason for my weak medium ships is that I never bother building small ships. They are just too weak/small to carry weapons. I only build medium, large and massive. Once I get massive, I see no reason in building large ships or smaller, since massive is superior in every way. But yes, I try to go for the massive ship type before really researching defenses and weapons. I recently built a nice massive ship with anti-missile and laser defenses (since enemies tend to steer clear of railgun-type). They are practically indestructible (1 ship was slightly damaged after tough firefight). I also found that playing on a huge/gigantic map is easier since enemies wait longer before attacking you and they also have trouble attacking period because of the large range. I'm doing well now with 400 income and 100 percent industry. Working on researching weaps. I think I have enough massive battleships to defend myself for a while. I also have 96 soldiering, which is nice.
Btw, I was playing on Normal. I think my problem was that I played on small maps too much and it was very difficult. On large maps I can just waste time researching and delay the other factions by paying them off. I'm currently allied with Drengin, Torian and those green guys against the Altarians, who aren't packing that much firepower (even my medium-sized obsolete ships can stand up to them). Just conquered a minor faction btw. They had full starport of large-scale/med-scale, but my 3 massives ripped through them (had fully upgraded battle space station nearby to help).