I want to ask you if slow Planets (Neptune, Pluto, Uranus) can be considered part of a Stellium. Let's say a person has Uranus Neptune and Venus in one Sign, and this sign is Aries, for example. Has he a strong influence in Aries or not? And if there is only a slow Planet and the other two are fast?
I'm talking about Charts where Neptune, Uranus and Pluto aren't prominent.

I think it can be but it would have more pronounced effects if it was made a personal planet like for example being a ruler or co ruler of the Chart.

I thought planets had to be conjunct to be considered a stellium to be honest though.

In this case the person may have more Aries traits yes but if all this was conjunct it might be more pronounced since Venus is there and it’s a personal planet.

It might not be as strong as say Mars Jupiter and Venus in Aries but the effect will still be there.