Almost every front loader has a pause, so yes you can add clothes after starting the cycle. However I would not add anything after the first few min. to get full cleaning.

There is no "critical" rpm, however don't let the BIG numbers scare you. Most wrinkle free fabrics shouldn't be spun over 800 rpm's. Some FL's that advertise huge spin speeds, you will typically only use these very high speeds (1000+ rpm) for towels, jeans, sheets...durable materials.

Many brands use very high speeds as a step up feature, however it is rare to get full use of them.

I hope this helps.

Ditto joelcoffey.

Don't get too excited about spin-speed claims. Anything you buy will spin out your load just fine. I have Duet 9400 but regard as non-issue with any of the new machines.

Almost every brand will allow you to pause and add clothes to a warm or cold cycle. Mine will not allow me to add clothes when the water temperature is either "hot" or "sanitary". It's a safety feature on my LG Tromm. I understand the reason for it, however, it's very aggrativing to find the stray white sock after the wash has started only to discover that the machine will not unlock to allow you to throw it in.