Well, there are 2 issues here.
Firstly, the look of the ceremony where there is an uneven balance of attendants for the Bride and Groom. If there is an imbalance of only one extra groomsman, that will not be noticeable during the ceremony. However, after the bride and groom have been announced as Mr and Mrs, and are walking down the aisle, it is normal for pairs of attendants, one pair at a time, to follow the bride and groom. This can't happen with an uneven number. Normally the last attendant walks alone. It does not look strange, particularly if the attendant has a smile for the walk.
If there is an imbalance of 2 groomsmen or bridesmaids, sometimes an even split of people, irrespective of the gender is nice while the ceremony progresses. Remember to have the ring bearer closest. However, I have officiated a number of weddings with imbalance of numbers, and it has not been an issue. The walk leaving the ceremony could have both walking together (probably without hand on forearm).
As far as the bridal waltz, the tradition is for parents of the bride and groom to follow the married couple. This is then followed by the rest of the bridal party. Where there is an imbalance of numbers here, one of the men can ask his girlfriend or wife to dance. Honestly, by the time the fourth groomsman enters the dance floor, the guests will be watching the bride and groom exclusively.