Could a same sigil, be activated/charged, more than one time, through different charging methods, before forgeting it?
I assume you are talking about a Spare style sigil made from a statement of intent?
If so, the answer is yes. There are no rules when it comes to sigils.
it is a Spare sigil but created on a magick planetary square. Does it make any difference?
No, it does not.
Do other seals have rules? if yes, what are they?
Planetary or spirit seals are not quite the same as Spare style sigils. They invoke specific types of external energy to bring about a material result, whereas a Spare style sigil is meant to be thrown into your subconscious mind, and because they draw upon an external source, they do not require one to forget about them for them to manifest.
However, other than that, there isn’t really any hard and fast rules when it comes to sigils or seals. While some methods of using seals are considered traditional (there are systems of magick that claim seals can only be used in the traditional way), they can also generally be used in many of the same ways as Spare style sigils.
The only way to find out is to experiment.