I think it depends on the child. Some have no problem with it, and some do.
For me, we all co-sleep in our bedroom, with a futon mattress on our floor, my son in the crib, and my daughter likes to sleep with us in our room (at night only). When my son wakes during the night, I get him, co-sleep with him on the floor futon, and he's fine with my daughter there too. BUT, he did go through a phase where he DID NOT want to sleep with his sister. So that was respected and we didn't force him. We explained to my daughter. It was fine. Now though, it's no problem.
Since your eldest is a toddler, I would speak to her about it first. Don't just "Force" it on her. Some kids don't sleep well with a noisy/waking/crying little sibling in the same room as them. Main thing being, you want the children to have a good night's sleep. You don't want one child keeping other awake. That is no fun.
As for what time to put them to bed, we would put my son to bed first... once he fell asleep, then I would enter the room and put my daughter to sleep. As they got older, then I and my kids ALL go to bed at the same time. My son and daughter LOVE sleeping with each other...now. It wasn't always like that. So, keep in mind, that "phases" may ebb and flow... and it's something they may like or not. But in either scenario... my kids like for me to be with them as they fell asleep. Thus, co-sleeping. But since that is not your situation, then that is one less "transition" you have to deal with.
It will be a "transition" no matter what. Just try it. See how it goes perhaps. BUT if it doesn't work.. then don't force it. Nothing is harder than for a young child and baby not being able to sleep because of each other. My son went through a "phase" where he would kick and push my daughter out of his way while they slept & tell my daughter to go away... but my daughter was okay about it because she said "it's okay Mommy, he's my little brother, I don't mind...." But fortunately, he passed this phase.
ALSO, we have ALWAYS had a routine pre-bedtime. ALWAYS. Our kids know it like the back of their hand. There is no protesting about it. We are not hard-handed about it...but they KNOW their bedtime routine like being on auto-pilot. This helps any child. Keep consistent about it. It will then be less of a struggle... and keep it up, despite the NORMAL developmental changes they go through.
If you can have your kids in their own room, I would do that. If you don't have to move your baby out of your room yet...then wait. My son is still in his crib, in our room, and sleeps well there, and he is 28 months old. We have no rush about getting him out. And at night, we all co-sleep anyway. My son, loves his crib though, and is consistent in it, and does not try to climb out or anything.
Being that your baby is only 5 months old.... YOU will STILL be needing to wake for baby, feed/nurse the baby, get up for any night wakings or cryings etc. It should not be a "burden" for your Toddler. And your Toddler is sleeping well at this point. That is great. If you don't have to stir the pot, yet... then I would wait. A 5 month old baby is still young... and you'd still have to have a baby-monitor or listen for the baby whenever she wakes. She is still a young baby... you will then have to get up, go to their room, get the baby BEFORE it wakes your toddler, feed/nurse baby, put her back in the room, get her back to sleep (or not) and deal with all of that. A baby this age, is going to guarantee have MANY more phases and night-time wakings and growth spurts and teething episodes yet to come. So, is this "convenient" for your Toddler, who is sleeping well now, and needs sleep?
Get a crib, put in it your room, and baby will be more reachable for you there... while not disturbing your Toddler. Then by the time you CAN move your youngest into the "same" room as your eldest... the age-phases and maturity of your eldest, will perhaps be more conducive to the combing of them into 1 room.
Good luck,
Susan