Hi
Guinea pigs are territorial; you always have to conduct the bonding in a place that is not regularly used by any of the parties involved. You can use the new cage as a bonding pen, provided your new arrival hasn't previously used it. I would not use the bathtub as your boy could be territorial about it if he has been a regular sole user.
If you want to go for a mixed gender pair, then one party needs to be de-sexed. Be aware that boar neutering only takes away the ability to make babies, but does not change personality or outlook in life and does not change their behaviour. Any bond stands and falls with mutual liking and character compatibility, and there is nothing you can really predict how the dynamics between the two will develop.
You may find the additional information in these guide links here helpful, as they provide an in-depth look at aspects we cannot expound all the time in a single post or thread, seeing that we are all doing this for free in our own free time. Please take the time to read our links.
A Closer Look At Pairs (Boars - Sows - Mixed)
Rescues, Shops, Breeders or Online? - What to consider when getting guinea pigs (also includes a chapter on rescue dating and links to recommended good welfare standard guinea pig rescues for several countries)
Insecure piggies can over-react with fear-aggression, which is becoming an increasingly more common issue due to the indiscriminate mass breeding for pet shops or for-sale breeders. A piggy being tame and cheeky with humans may react very differently when faced with another guinea pig. It is very difficult to predict the reaction. Age and gender come generally a long way behind character compatibility. A bonding is generally easier with a sub-teenager (under 4 months) because at that age, youngsters very much rely on companionship and guidance from an elder and are much more accepting. Sub-adult sows are often more accepting of a boar because they have the biological behind them - but a happy bonding is not guaranteed even in that case. With baby boars you have to then navigate teenage and with spayed sows (who are usually around 6 months or older) you may be facing fear-aggression issues. A cross gender intro is not - other than most people seem to think - not necessarily fail-safe. I've got plenty of t-shirts!
If you are lucky to have access to a good guinea pig rescue that offers dating for your single or bereaved piggies, then please go down that route; it may take longer, involve travelling a distance and not be quite as cheap, but it is well worth it - including support in case the bond is not working out. Otherwise you have to be able to offer adjoining cages so each piggy can have their own territory but have all the vital stimulation via pheromones, body language and vocal interaction that a social species like guinea pigs needs as your fall-back option.
I hope that this helps you?
I love seeing everyones cages for them 
