I have a m33 foregrip with a the TDI short vertical grip right now. I just ordered a TD grip for my new project to replace the m33 with a Surefire m73 and to replace the TDI w/ a TD. Since I am young and have little cash flow I bought the TD grip first with hopes to attach it my m33 while I earn money for the m73. Today I realized that the m33 is slightly off spec. Will my Tango Down grip still work? Will I have to file down some of the rail? If I have to file it down will it decrease the value of the m33 very much?

I don't think you will have to. I have the M33 and both the KAC grip and Dieter grip work fine with it.

I have the M44 handgaurd... it's the same as the M33 with respect to the lower rails.., and I have a TD VG on it. It does fit really tight... you might need an extra pair of hands and some CLP to help slide the VG on.