Thanks for the additional information. It is helpful.
Daisy does need some obedience training at a minimum. All dogs regardless of breed, small, large, etc should have obedience training to allow you to control their actions without having to physically grab them. In addition, it helps when you have multiple dogs to have them obedience trained.
The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Daisy has matured into an adult and with Suzie having medical issues, Daisy would be the logical choice for top dog even if she was not dominant by nature. So if Daisy started thinking she was in charge and did things like walk through a door first or push past Suzie to get to you or even eat first, Suzie might go to reprimand Daisy even if it was just to push back or a growl or just get close to her. Daisy, now that she is mature may have decided that she was going to reprimand Suzie with a bite. Of course, Suzie would fight back.
I am sorry you had to have her put to sleep. It is always traumatic to lose a family member. We recently had to euthanize one of ours so I know it was a hard decision. You definitely want to get a male dog and one that is pretty easy going. I would tend to get one just a little larger than Daisy if it was me. That would help minimize any injuries if a fight ever was to occur. Since she is not dominant by nature, , she should not have a problem especially with a male dog. Now if you do go for a labrador, I would try for one 3 years old or more and well behaved. I'd also take your girl to where ever you get him at so you can see how they interact on his territory. I'd also try to arrange a few of these type visits before bringing him home. That way they can get used to one another as well before he enters her territory.
I'd also crate him a good part of the day initially so she gets used to his presence and smell. I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.