Thanks for your post, and welcome to Ultralight DX fun.
The designers of the slider E100's (John Bryant and I) have experimented with coupling external antennas to the slider loopstick ferrite bar by using an inductive coupling system, where a BNC connector runs to a small coil of wire on the ferrite bar, on the side opposite the coil. This not only allows the effective use of high gain external antennas like EWE's, K9AY's and beverages, but the slider coil can still be used as a preselector, filtering out unwanted image and spurious signals.in urban and suburban areas. In such cases, the high gain of the external antenna "swamps" the slider loopstick RF pickup, so that there is no incompatibility or "competition" between the two antenna systems. John's fine article posted on dxer.ca., "Slider e100 as a Varicoupler" gives full details.
To my knowledge, nobody has yet tried this slider E100 inductive coupling system with a smaller antenna, like the Quantum QX-Pro Loop. This is one area of experimentation that is still uncertain. The 7.5" loopstick slider E100 by itself is capable of great sensitivity, as any owner knows. During summer experimentation, experimental 17", 22.5" and 27" ferrite bars generally didn't provide the hoped-for great sensitivity benefits over the 7.5" models. After building your slider E100 and testing it out with the Quantum QX-Pro Loop, you may also discover that the 7.5" model provides a lot of "bang for the buck." But we certainly would be interested to know your results, either way. Good luck on your slider project.
73, Gary DeBock