Dosing full EI doses, with a partially grown plant load, will not cause problems if you always do the 50% weekly water changes. Those water changes keep the buildup of ferts within a safe limit. In my bigger tank I have always dosed full EI doses (usually 2X the EI dose of KH2PO4) and I have seen no problem caused by that, even when I have just finished a major pruning operation.

1.7 watts per gallon of T5 light is more than enough for growing almost all plants. Those are very efficient watts.