size_t best practice

Many C std. lib. functions accept and/or return size_t values esp. for
arrays incl. char* strings.

In view of this I'm using size_t throughout my code for array sizes and
indexes.

However, this means I have to be very careful never to decrement a size_t of
value 0, since, e.g., size_t size = 0; size--; results in size ==
18446744073709551615.

So I need to guard against this. Here is an example I'm using
(without the assert()s):

void vec_insert(vec* v, size_t index, void* value) {
    if (v->_size == v->_cap) {
        vec_grow(v);
    }
    for (size_t i = v->_size - 1; i >= index; --i) {
        v->_values[i + 1] = v->_values[i];
        if (!i) // if i == 0, --i will wrap!
            break;
    }
    v->_values[index] = value;
    v->_size++;
}

I've also noticed that quite a few array-related algorithms _assume_ that
indexes are signed, so again I have to put in guards to avoid subtracting
below zero when I use size_t when implementing them.

So is it considered best practice to use int, long, long long, or size_t,
in situations like these?