So i have a 8 spped cassette w/ a 7 speed derailleur using non-indexed friction shifters. Now with proper H/L screws adjusted, i can reach all 8 cogs on this 7SPEED derailleur. Slight problem with this is that it slips on the smallest cog. So would the problem be that im running a 7speed derailleur on a 8speed cassette even though the derailleur reaches the smallest cog...

And chain is an 8speed chain as well.