Ruth has given you lots of really good advice and said everything I would have and more on those two subjects!
I did Slimming World for about 6 months and only lost about a stone in that time. I think you would have to think carefully about making it work with low GI but it could be done. I did Extra Easy when I did it but if I were trying to do what you describe I think I would:
Only do red days, and try to keep most of my HEXB choices away from being bread and avoid entirely any of them containing sugar/sweeteners. Use my syns (15/day) on healthy items containing good fats: nuts/nut butters, seeds, olive oil, olives, avocado, etc. Completely avoid using my syns on anything processed - crisps, curly wurly bars, etc. I would use some for a small amount of dark chocolate a few times a week, though, or a splodge of St Dalfour sugar-free jam. Always make sure that I had protein and carbohydrate at every meal and snack, and ensure I ate something every 3-4 hours. SW seems to promote having 3 large meals and trying to avoid snacking but GI/GL espouses having slightly smaller main meals to allow for a morning, afternoon, and even an evening snack if you need one. Although you don't need to by Slimming World's rules if following red, I'd also try to make sure I had at least 1/3 superfree vegetables with lunch and dinner and preferably my one piece of fruit (or some superfree veg like mushrooms/tomato) with breakfast.
Some of the things that 'everyone' on SW seems to do that I would try to avoid: eating loads of fruit - as I'd be having 2 carb servings already as my HEXB choices, I'd try to limit to one piece of fruit per day. Artificial sweeteners and the things containing them (Muller Lights, diet Coke etc). Not such a problem on red, but processed snack foods that are free on green/EE, like Mugshots, fat-free supernoodles, etc. The much-praised 'unlimited' portions - personally when I was on SW I found it really hard to know when I was full, and would just eat what I had prepared and then discover afterwards that I could have easily stopped after eating 2/3 of it.
Do you have any of the older SW recipe books that have red/green recipes in them? As a lot of red day recipes are naturally low GI already. If not you can often pick them up on eBay quite cheaply (and Amazon have some of the ones that were printed for the general public, often with cheap copies available from Marketplace sellers).
Personally, I found that my body didn't really start to respond well until I switched to low GI/GL AND figured out the number of calories I needed to be eating. With SW it was too hit and miss, I could follow 'the rules' 100% and still have a huge variation in how many calories I ate. The calorie watching is a pain at first but you quickly get to learn what a meal looks like if it's something you have often. But if you're already familiar with SW principles and you want to start more easily by incorporating low GI principles into that plan as quickly or slowly as you feel comfortable with, and that works best for you, then I'd say go for it and see how it works for you!