Just to clarify something that may or may not already be understood: the SL66 and SLX use different lens mounts; lenses are not directly interchangeable between them. If you're looking to adapt and aren't interested in the camera bodies then it's important to note you'll need a helicoid adapter to use SL66 lenses on any non-bellows camera, as the SL66 uses a bellows for focusing. SLX and 6000-series lenses have helicoids and so an adapter would be more affordable. The SLX has a much shorter flange focal distance, to the point where it is not possible to adapt the lenses to Pentax 67. The same limitation applies to Hasselblad lenses - you will not be able to adapt them to Pentax 67.
Adapting SLX or Hasselblad lenses to Pentax 645 would be possible but would require a fairly thin adapter; I am not sure if one exists. The SL66 has a very long flange focal distance, so everything short of the 120mm is retrofocus. The lenses can be used on just about any other SLR system as a result, with the reminder that the helicoid caveat exists. One other difference applies if we get into coated lenses (so SLX and later SL66 lenses), which is that Rollei used their own HFT coating instead of Zeiss' T* lens coatings.
Since you mentioned possibly getting into a Rollei system it would be worth doing some more thorough research into the SL66, SLX, and 6000-series camera before making a decision. There are substantial differences between the camera lines, and even more dramatic differences when compared to Pentax medium format SLRs. It's worth exploring the question of why, as well - Zeiss glass is great, but so is Takumar glass. I don't personally think it's worthwhile to adapt lenses across existing viable systems most of the time; the fun of adapting is in using lenses that can't be readily used otherwise. From that perspective, the SL66 is a wonderful camera even aside from the system lenses.