I can try to answer your questions, though others may have more experience. We used one of Sonlight's preschool cores last year and will be sticking with them for Pre-K this year. So far, I love Sonlight, but I agree-their system is a little hard to understand at first glance.
I am not sure what ISP is (new to homeschooling), but when you say you want to supplement with Sonlight do you mean that you only want their literature and not subjects like math, handwriting, science etc? With Sonlight, their "core" contains read-alouds, history, and Bible. With cores above the third grade level they also include readers and language arts. I am actally not sure whether you have to order a set of readers/language arts when you order a lower core, but if you do, you also have to select the child's reading level.
Basically, if you want everything for a full year of study (including all subjects) and want a quick, easy order option, go with one of their Multi-Subject packages. For those, I think you have to choose the core that they list for that particular "grade level." So, if your daughter will be doing first grade, you would order the multi-subject package for core B.
If you want to customize your order, go with one of their cores and then build on however much you want. Under each core, there is an age range listed. A lot of people on the Sonlight forums seem to say it is best to stick to a core that lists your child's age on the middle or upper end. So, if you have a six year old, I would personally go with core A (K), though core B (previously 1st) would be fine too. The grade level labels are confusing, because the cores themselves are not confined to one grade. If your daughter is seven, you could do anything from core A to core C. Personally, I would probably pick B in that case.
You also have to add on readers and language arts. Pick the grade level that fits your child for that. Then, add on any subject you want-choosing whatever program you want.
I know it can be confusing. I am actually glad I am using them for preschool so I get to know the ins and outs of their system before tackling the school-age cores. They do have great customer service, so if you give them a call or ask them online, I'm sure they can help. Let me know if I can answer any more questions. I hope I haven't confused you even more.