1st generation RSX (1995-1998) was both Uniglide and Hyperglide compatible. You can see the threading for the outer cog in the atttached 1995 catalog scan. It also shows a Hyperglide cassette as standard. The 2nd generation, introduced in 1999 was 8 speed and Hyperglide only.

Hyperglide cogs improve shifting under load whether it's friction or indexed. I always go with Hyperglide, where possible.

One caution is that while 6 and 7 speeds hubs have the same OLD (Over Locknut Dimension), the 7 speed cassette/freewheels are slightly wider, with the outer cog typically sitting closer to the dropout. Consequently, the cogs and/or chain will sometimes interfere with the stays on frames designed for 6 speed if they not been relieved on the inside surface. This can generally be solved by using a 1-2mm washer under the drive side locknut, though the axle should be re-centred to ensure equal protrusion beyond the locknut on both sides.