Free Sprite/Object slots

I want to either replace decorating object sprites with Npc Chars and signs that will display text *Step on sector*, or make my own objects.

The wiki is not very clear on how to do this, Anyone have any suggestions on what I can do or Where I can find info?

Also I would like to do this all in one Map wad.
if that is possible

Alsoo, with Cecho, How many Text Chars can be show on screen without cut off.
I could test it out, but I might save half an hour if one of you know.

IIRC, the amount of text can be cut off sooner or later depending on screen resolution or aspect ratio. It probably would be best to make it in compatibility with the game's smallest resolution setting (320x240?) in mind.