Should I fix the gap between my lower front teeth?
If it improves your self-confidence, yes.
But only if it can be done with braces and does not affect the crucial bite pattern of your other teeth.
Fortunately, the spacing between the lower incisors is usually not a major aspect of chewing food. Tooth gaps (diastema) are more common with the upper incisors.
Usually, the upper front teeth comes down just in front of the lower front teeth. Underbite is when the lower front teeth are in front of the upper teeth. Underbite is usually described as more, or less. Possibly bigger or smaller underbite. Talking about a stronger underbite doesn't quite fit. But it should be obvious that bulldogs have more underbite than than Rottweilers. A Rottweiler shouldn't have any underbite at all.
if your tongue is between your front teeth when you suck it, you get an open bite. your front teeth can also stick out. i suck my tongue too but instead of it between my teeth, i fold my tongue in half and then suck it. that caused me to have an overbite because somehow in folding my tongue and sucking it, it caused my lower jaw to go in hence my overbite.
Your hamster has lost it's two upper front teeth Will the lower teeth be overgrown to cause any complication to the hamster?
Hamsters are part of the rodent family. The two top front teeth should grow back. However, if they do not, you need to consult a veterinarian. It may complicate some things for a little while, but feed it soft fruits and veggies and it should be fine until it's top teeth grow back. Also, get the hamster a chewing stick so that he can keep his lower teeth from becoming overgrown.