Up until now didn't care for the storage engine and left the default MyISAM (thought there must be a reason for making it the default), I always used through my php scripts columns with the same data in different tables in order to connect them but now since lots of my sites' section are going into my MSc thesis I am wondering if it would be wrong not to actually declare them as foreign keys and add the restrictions, and if it is such a big deal it is easy to change from one type to another? Simply change the type and everything will work like before?
P.S. We are talking about tables with small to moderate size (do not thing anyone will get above 100 records maybe only the users' and products' table), and the select are much more common than the insert/update statements