Re: Finishing PM6.
Unless you've run into the problem of the series not working any more, I'd vote to finish it.
We used Math Mammoth, not Singapore PM, however I had both my kids do the 6th grade level. The older version of MM that we used was 1-6, and hadn't been revised to include 7. Being the last book in the series, its the level where they will get to learn new topics that hadn't been introduced yet and more importantly, got to practice problem solving on problems that synthesize and bring together all the skills that they've learned in the last couple of levels. They are AL, so it's not like we're racing the clock.
The way that I viewed it, the 6th level of a 1-6 series isn't extra it is the end. I think we did MM6 in a few months and I think that Singapore/Primary Mathematics is similar to, or possibly shorter than MM6.
Re: Algebra with young ALs.
It's been a while and my memories not perfect, but we did a lot of elementary algebra using a mish-mash of resources that I already owned along side MM5 and MM6. They didn't have any real problems, I think maybe with setting up word problems in the middle and then quadratics, towards the end?
After they finished MM6, we used a real text and did a "fast-pass" over view/run through of an Official Algebra course. I wanted to see what they knew well and what needed more work and over all they did great with the material, but they still weren't ready for a BIG textbook at 7ish years old.
Re: AoPS with younger kids
I'd caution you give it a try if you want too, but for us, AoPS has ranged between being "meh" and being a "flop". We tried 3 different books and they can do the work, but they just don't like the books. Some kind of way mine are the kids who love math, are good at math but just don't like AoPS.