It's been said before, but it's worth mentioning again, that working with saints is not a common practice in Hoodoo. The majority of rootworkers are Protestant, which explains why the veneration of saints is not a common feature. Online, working with saints in the context of Hoodoo is disproportionately represented. Oftentimes the "working" relationship with saints is given more attention than the devotional relationship with a saint, often removing them from their Catholic or Orthodox cultural contexts.
See also these posts:altars-questions-and-answers-t124s270.html#p83541newbie-question-about-working-with-saints-t450s0.html#p322questions-about-candles-for-saints--t5810s150.html#p33374hello-help-with-casting-and-creating-your-own-spell-t7190s120.html#p41366
This is of course not to say that non-Catholics cannot petition the saints: anyone can. However, one should expect (as Miss Cat mentions in one of the above posts) that a non-Catholic would have a deep appreciation and respect for the Roman Catholic (or Orthodox) Church and be willing to learn about the veneration of saints from someone of that culture. As an aside, I'm not singling you out Caedryn; you may be Catholic for all I know. I'm just writing for everyone's benefit.
Since you are new to petitioning saints, I encourage you to familiarize yourself with the role that saints play in the Catholic Church, particularly the doctrine of the intercession of saints. Take advantage of the wealth of information about the saints available online: the Catholic Encyclopedia and Butler's Lives of Saints
are both great places to start. Speak to Catholics about how the saints have impacted their lives; many Catholics have an interesting family story there!
Understand that petitioning the saints is one part of the larger Catholic folk magic tradition. Learn from Catholic folk magic practitioners about the use of priestly blessings, sacramentals like holy water, traditional litanies and novenas, the use of different prayers like the Rosary, etc. That's really the best way to learn about and work within a certain tradition: from the people who are a part of that tradition.
The reason I'm saying all of this is because working with the saints has increasingly taken on a "gum ball machine" approach. Have a problem? Petition such-and-such saint, get what you want and move on. Again, I'm not suggesting you would this, but it is all too often the case. So familiarize yourself with the Catholic tradition. If you find that you are unable to work within that tradition, then perhaps working with the saints is something not suited for you.
Now to get to your questions.
Most saints are fine sharing the same space on an altar. If I'm not actively working with a saint on my altar, I just leave them where they are. To me, an altar for the saints is not just a place where I perform magic, but it is also a devotional space where I engage in spiritual practice. Depending on your altar space, you may be able to undertake the work or "spell" directly in front of that saint's statue or icon while still having enough room for other saints. If not, adapt to your space needs.
There are sometimes exceptions. As a personal example, Santisima Muerte and the Archangel Michael each have their own dedicated spaces. This is because I believe them each to be such powerful and holy spiritual beings that they deserve their own spaces for honouring them.
If you decide that your relationship with a particular saint has run it's course, the traditional and respectful way to pass on their statues/icons/prayer cards/etc is to take them to a Catholic church and leave them tucked under the pew or in a corner of the church. There is separate protocol to be followed if the item was a sacramental to prevent desecration.
Finally, in between workings, I love to light a single white candle dressed with Lucky Mojo All Saints oil, as an offering to all the saints I have a relationship with.
Sorry for the long-winded reply, Caedryn. My intention is not to sound "dogmatic" as it were, or like I'm policing how people work with saints. I sincerely hope you, and anyone else new to the saints find it useful.